1. Field of the Invention
The invention is directed to multicore optical fibers. More specifically the invention is directed to methods of designing multicore optical fibers in which every core can be inscribed or addressed simultaneously without blocking adjacent or other cores.
2. Description of Related Art
Multicore fiber gratings are well known. They have been used in fiber sensors for bend and shape, for example. It would useful to use such gratings in other applications such as telecom and multicore fiber lasers. However, such gratings are more demanding. They require well controlled exposure over a long length of fiber. Moreover it is most important to be able to fabricate gratings in parallel in multiple cores since this would greatly increase yield for densely integrated multicore fiber devices employing fiber gratings.
The prior art considers fibers with a limited number of cores, typically four or fewer. In order to scale fiber designs to more than four cores the fiber design must be adjusted. Moreover the precise orientation of the fiber must also be adjusted for optimal exposure of all cores. Such optimization is particularly important in round silica fibers, since these fibers exhibit lensing of the incoming light. Such fiber designs and fiber orientations and their use in parallel fabrication of multicore fiber gratings have not been disclosed.
Some areas of interest for multicore fibers include image transmission, telecommunications, sensing, and fiber lasers. Recent results have shown the possibility of long distance propagation of spatially multiplexed telecom signals with low cross talk over seven core fiber. MCFs have been used as sensors of temperature, and strain, as well as fiber bend and shape. Nonlinear effects, including switching in dual core fibers have been examined. Multicore rare earth doped gain fiber with various geometries has been demonstrated. Coupled twin core fibers have been considered for Er doped amplifiers and lasers. Fiber lasers operating on a supermode of many coupled cores have been proposed and demonstrated. Gain fibers with uncoupled core geometries have also been considered. These designs are motived by the desire for increased integration in telecommunications, sensing and fiber lasers. Improved diode pump coupling and scaling of fiber laser output power has been demonstrated in multicore ribbon fibers. Seven core hexagonally arrayed Er doped fibers have been applied to telecom signal amplification with low cross talk among the cores.
While past work has shown multicore lasing and amplification, these results have typically employed fused fiber and bulk optic components for filtering or to construct laser cavities. There is comparatively little work reported on multicore fiber Bragg gratings (MCFBGs) as components in integrated multicore fiber sources. Multicore fiber gratings have been demonstrated in many of the above fiber sensors, however such gratings are typically less demanding than FBGs used for fiber lasers. For instance, fiber distributed feedback (DFB) lasers require well controlled holographic inscription of intra-core index modulation over lengths of cms with a precisely placed it phase shift to define the cavity. In single core fibers, it is well known that fiber DFB lasers can exhibit sub MHz linewidths. Extension of narrow linewidth fiber DFBs to multicore fibers could impact multicore fiber sensing applications, particularly those using interferometric or RF interrogation. Precision MCFBG fabrication could also impact the development of compact high brightness multicore fiber lasers. Moreover, if such gratings could be fabricated in parallel in all cores of a multicore fiber, an important efficiency in fabrication would be realized. Scaled fabrication and assembly of multicore fiber devices is an important driver for research into multicore fiber technologies, since it offers the possibility of reduced cost and size in next generation fiber components that require dense integration of many fiber cores.
Accordingly, there is a long felt-need in the art to provide a method of designing multicore optical fibers, e.g., having seven or more cores, in which all of the cores are able to be inscribed with gratings or the like simultaneously with a single inscription beam and/or addressed simultaneously with a single addressing beam. There are also long-felt needs to provide optical fibers so designed, and devices such as lasers and pump couplers utilizing optical fibers so designed.